As a rule, blood purification machines serve for use of an extracorporeal blood purification replacement treatment (for example dialysis) especially in the case of renal failure. Apart from renal transplantation, such blood purification replacement treatment or dialysis is the most important renal replacement therapy in the case of chronic renal failure and one of the possible treatments in the case of acute renal failure.
By the term “dialysis” an exchange of substance via a membrane arranged in a dialyser is understood, wherein on one side of the membrane blood/plasma is present and on the other side of the membrane the purifying fluid or else dialysis fluid is present. Concretely speaking, blood is guided out of a patient in an extracorporeal blood circulation to the dialyser of the machine and there is passed by the membrane. On the machine-side, preferably water suited for dialysis is withdrawn from a reservoir or an external line, is processed to form a dialysis fluid in an internal fluid circulation and the fresh dialysis fluid is equally passed to the dialyser so as to absorb contaminants from the blood flowing past.
In so called hemodialysis the principle of adjusting the concentration of small-molecular substances of two fluids is applied which are separated by a semipermeable membrane in the dialyser (osmosis). In this case, separated by the filter membrane, on the one membrane side the blood discharged from the patient's body including nephrotoxins, electrolytes (especially potassium and phosphate) as well as substances usually eliminated with the urine is provided. On the other membrane side the low-germ possibly processed dialysis solution (dialysis fluid) is provided, wherein the water thereof was processed, in case of need, for instance during online processing by reverse osmosis or other known measures, which is initially free of waste products and which includes a content of electrolytes oriented at the particular needs of the patient.
The semipermeable filter membrane (dialyser membrane) between the blood and the dialysis fluid necessarily has pores that allow small molecules such as water, electrolytes and substances usually eliminated with the urine (e.g. urea, uric acid) to pass but withhold large molecules such as proteins and blood cells.
Before the dialysis fluid or purifying fluid in the dialyser flows past the patient's blood, it has to be heated from the original entering temperature of typically approx. 10° C. to a higher temperature, approximately equal to the blood temperature (about 37° C.) so as to avoid cooling of the patient. After the dialysis fluid has flown through the dialyser so as to absorb contaminants from the blood, it is deemed to be used and is discharged in a drain for later disposal.